Advances in turbine engine aero- and thermodynamic performance have led to increasingly larger thermal gradients across turbine engine rotating components. Larger thermal gradients, as well as the turbine engine duty cycle, induce high cycle fatigue (HCF) and low cycle fatigue (LCF), ultimately reducing the structural life of rotating components. Additionally, thermal gradients are at least partially responsible for thermal bowing, or bowed rotor, in rotating components such as shafts, due to asymmetric cooling following turbine engine shutdown.
HCF, LCF, and bowed rotor may result in rotating component failure during turbine engine operation or necessitate replacement of the rotating components at smaller intervals, thereby increasing turbine engine operating costs. HCF, LCF, and bowed rotor may also induce damage to other turbine engine components, such as bearings and casings, due to deformation of adjacent rotating components, which may result in decreased turbine engine efficiency, performance, and structural life.
Increasing rotating component thicknesses, using denser materials, and directing cooler air from the propulsive thermodynamic cycle to rotating components are known to reduce thermal gradients, thereby increasing structural life and mitigating bowed rotor. Additionally, instituting a longer cool-down period during turbine engine shutdown or between shutdown and restart is known to mitigate bowed rotor. However, directing air from the propulsive thermodynamic cycle and using thicker or denser materials reduces turbine engine efficiency and increases fuel consumption by reducing the energy available for, or requiring additional energy for, propulsive thrust. Furthermore, longer cool-down regimes increase the amount of time before the turbine engine can be restarted, which may have an economic affect similar to reduced engine efficiency or increased fuel consumption.
Therefore, there is a need for reducing the thermal gradient across rotating components while mitigating adverse effects to turbine engine performance, efficiency, and economics.